api: add nvim_buf_call to call function with curbuf changed to buffer

src/nvim/api/buffer.c

@@ -1666,6 +1666,44 @@ Integer nvim_buf_set_virtual_text(Buffer buffer,
   return src_id;
 }
 
+/// call a function with buffer as temporary current buffer
+///
+/// This temporarily switches current buffer to "buffer".
+/// If the current window already shows "buffer", the window is not switched
+/// If a window inside the current tabpage (including a float) already shows the
+/// buffer One of these windows will be set as current window temporarily.
+/// Otherwise a temporary scratch window (calleed the "autocmd window" for
+/// historical reasons) will be used.
+///
+/// This is useful e.g. to call vimL functions that only work with the current
+/// buffer/window currently, like |termopen()|.
+///
+/// @param buffer     Buffer handle, or 0 for current buffer
+/// @param fun        Function to call inside the buffer (currently lua callable
+///                   only)
+/// @param[out] err   Error details, if any
+/// @return           Return value of function. NB: will deepcopy lua values
+///                   currently, use upvalues to send lua references in and out.
+Object nvim_buf_call(Buffer buffer, LuaRef fun, Error *err)
+  FUNC_API_SINCE(7)
+  FUNC_API_LUA_ONLY
+{
+  buf_T *buf = find_buffer_by_handle(buffer, err);
+  if (!buf) {
+    return NIL;
+  }
+  try_start();
+  aco_save_T aco;
+  aucmd_prepbuf(&aco, (buf_T *)buf);
+
+  Array args = ARRAY_DICT_INIT;
+  Object res = nlua_call_ref(fun, NULL, args, true, err);
+
+  aucmd_restbuf(&aco);
+  try_end(err);
+  return res;
+}
+
 Dictionary nvim__buf_stats(Buffer buffer, Error *err)
 {
   Dictionary rv = ARRAY_DICT_INIT;

test/functional/lua/vim_spec.lua

@@ -1330,4 +1330,29 @@ describe('lua stdlib', function()
       eq(false, pcall_result)
     end)
   end)
+
+  describe('vim.api.nvim_buf_call', function()
+    it('can access buf options', function()
+      local buf1 = meths.get_current_buf()
+      local buf2 = exec_lua [[
+        buf2 = vim.api.nvim_create_buf(false, true)
+        return buf2
+      ]]
+
+      eq(false, meths.buf_get_option(buf1, 'autoindent'))
+      eq(false, meths.buf_get_option(buf2, 'autoindent'))
+
+      local val = exec_lua [[
+        return vim.api.nvim_buf_call(buf2, function()
+          vim.cmd "set autoindent"
+          return vim.api.nvim_get_current_buf()
+        end)
+      ]]
+
+      eq(false, meths.buf_get_option(buf1, 'autoindent'))
+      eq(true, meths.buf_get_option(buf2, 'autoindent'))
+      eq(buf1, meths.get_current_buf())
+      eq(buf2, val)
+    end)
+  end)
 end)